feat: maintenance flow UX redesign — batch status hub, context strip, detail page upgrades (#85)

- Add BatchStatusPage (/flows/:id/batches/:batchId): per-target Start/Resume/View cards, progress bar, 5s polling while in-progress, completion outcome summary
- Add BatchStatusCard: handles not-started/in-progress/complete states with step progress for in-progress targets
- Add ActiveBatchBanner: amber banner on detail page when a batch is running, links to BatchStatusPage
- Add MaintenanceContextStrip: amber strip in ProceduralNavigationPage for maintenance flows showing target name, batch progress (X/Y complete), and Back to Batch nav
- Update MaintenanceFlowDetailPage: active batch banner, clickable run history rows with mini progress dots and outcome summaries, Run button loading state, post-launch navigates to BatchStatusPage
- Update ProceduralNavigationPage: renders MaintenanceContextStrip between top bar and content when tree_type === 'maintenance'; fetches batch progress once on mount
- Add batch_id filter to GET /sessions backend endpoint and SessionListParams frontend type
- Add /flows/:id/batches/:batchId route to router

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/router.tsx

@@ -25,6 +25,7 @@ const TreeEditorPage = lazy(() => import('@/pages/TreeEditorPage'))
 const ProceduralEditorPage = lazy(() => import('@/pages/ProceduralEditorPage'))
 const ProceduralNavigationPage = lazy(() => import('@/pages/ProceduralNavigationPage'))
 const MaintenanceFlowDetailPage = lazy(() => import('@/pages/MaintenanceFlowDetailPage'))
+const BatchStatusPage = lazy(() => import('@/pages/BatchStatusPage'))
 const SessionHistoryPage = lazy(() => import('@/pages/SessionHistoryPage'))
 const SessionDetailPage = lazy(() => import('@/pages/SessionDetailPage'))
 const MySharesPage = lazy(() => import('@/pages/MySharesPage'))
@@ -180,6 +181,14 @@ export const router = createBrowserRouter([
           </Suspense>
         ),
       },
+      {
+        path: 'flows/:id/batches/:batchId',
+        element: (
+          <Suspense fallback={<PageLoader />}>
+            <BatchStatusPage />
+          </Suspense>
+        ),
+      },
       {
         path: 'trees/:id/navigate',
         element: (